1. Field of the Invention
This invention relates to a solder dispensing tool and particularly to an improved tool for dispensing solder while using a pencil type soldering ion.
2. Description of the Prior Art
Great difficulty can be encountered in situations where very small electronic components must be joined using soldering techniques. This is particularly true when miniature components such as chip capacitors, thick-film chip resistors, unpackaged inductors, leadless semiconductor devices, small diameter semi-rigid coaxial cable, and so on, must be installed into electronic circuit assemblies. Many applications utilizing these kinds of miniature parts may also be circuits designed for service at radio frequency (RF) or microwave frequencies. When this is the case, it is usually imperative that the parts be located precisely and that the resulting solder joints be uniform in size and of high quality.
This can sometimes be accomplished by employing paste solder using reflor sodlering techniques. This requires that the paste be applied to the circuit substrate while accurately controlling both the quantity and location of the solder. The joint is then formed by heating the substrate from below using a hot plate or from above using an infrared source (as used in Surface Mount Device technology). However, if the substrate either cannot withstand the sustained high temperatures used during reflow or does not have sufficient thermal conductivity, conventional reflow soldering is not possible. As an example, many soft substrates (non-ceramic) will not be candidates for reflow soldering. Furthermore, reflow soldering requires that all of the components to be attached be of similar thermal mass. This is not always possible.
It is normal for all of a circuit assembly's solder joints to be formed simultaneously when using reflow techniques. However, particularly during development design efforts, it is necessary to be able to repeatedly and individually form solder joints for one part while not disturbing others nearby (rework). When a solder joint melts and solidifies over and over again, poor mechanical and electrical performance results. Individual, isolated point by point soldering can be used to eliminate this problem.
Though it is unusual to do so, one can use paste solder while using a hand soldering pencil. However, poor joint quality can result due to paste metering problems and the uncontrolled loss of solder on the tip of the iron when it is removed from the molten joint. Also, this technique does not allow rapid and multiple rework while insuring that the solder joint has high electrical and mechanical integrity.
A possible solution is to use a hand operated soldering pencil and small gauge wire solder. This will then allow the operator to apply the exact amount of solder needed and to the exact location required. The characteristics of each solder joint can be adjusted as needed. However, this kind of soldering leads to a classic problem, as follows.
The miniature electronic components must be handled delicately, placed with precision, and held firmly during attachment. The soldering pencil must be placed at the optimum location to form the joint and for as short a period of time as possible. The wire solder needs to be fed to the joint as it is formed with precise control over the quantity of solder and the rate and point of application. An individual operator will find it virtually impossible to simultaneously hold and accurately control these three items.
Dispensers for feeding solder to the tips of soldering irons are known in the art. It is even shown in U.S. Pat. Nos. 3,996,109 and 4,507,545 that there are solder dispensers that can be directly attached to pencil-type soldering irons. These permit one-handed soldering operation, of a sort. However, the solder is usally applied to the iron's tip rather than directly to the joint as is proper. The solder feed rate is difficult to control. And independent adjustment of the relative locations of the component, iron tip, and wire solder cannot be achieved. Finally, these solder dispensers are far more complicated in construction and more expensive than the invention disclosed herein.